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Origin of Life

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Origin of Life Origin of life An explanation of what is needed for abiogenesis (or biopoiesis) Don Batten Introduction The origin of life is also known as abiogenesis or sometimes chemical evolution. Life is based on long information-rich molecules such as DNA and RNA that contain instructions for making proteins, upon which life depends. But the reading of the DNA/RNA to make proteins, and the replication of DNA or RNA to make new cells (reproduction, the mark of ‘life’) both depend on a large suite of proteins that are coded on the DNA/RNA. Both the DNA/RNA and the proteins need to be present at the same time for life to begin—a serious chicken-and-egg conundrum. Thus, the origin of life is a vexing problem for those who insist that life arose through purely natural processes (physics and chemistry alone). Some evolutionists claim that the origin of life is not a part of evolution. However, probably every evolutionary biology textbook has a section on the origin of life in the chapters on evolution. The University of California, Berkeley, has the origin of life included in their ‘Evolution 101’ course, in a section titled “From Soup to Cells—the Origin of Life”.1High-profile defenders of ‘all-things- evolutionary’, such as P.Z. Myers and Nick Matzke, agree that the origin of life is part of evolution, as does Richard Dawkins.2 A well-known evolutionist of the past, G.A. Kerkut, did make a distinction between the General Theory of Evolution (GTE), which included the origin of life, and the Special Theory of Evolution (STE) that only dealt with the diversification of life (the supposed topic of Darwin’s 1859 book).3 It is only recently that some defenders of evolution have tried to divorce the origin of life from consideration. It’s probably because the hope of finding an answer is rapidly fading, as one scientific discovery after another of sophisticated machinery in even the simplest living cells makes the problem of a naturalistic origin ever more difficult. So, what do we need to get life? We can break the problem of the origin of life into a number of topics in an attempt to explain to non-scientists what is involved (although it still might be mind- stretching). How did life begin? Explaining the origin of life by solely physical and chemical processes is proving to be extremely difficult. What is it that we have to obtain to produce a living cell? A living cell is capable of acquiring all the resources it needs from its surroundings and reproducing itself. The first cell had to be free- living; that is, it could not depend on other cells for its survival because other cells did not exist. Parasites cannot be a model for ‘first life’ because they need existing cells to survive. This also rules out viruses and the like as the precursors to life as they must have living cells that they can parasitize to reproduce themselves. Prions, misshaped proteins that cause disease, have nothing to do with the origin of life because they can only ‘replicate’ by causing proteins manufactured by a cell to become misshaped. The first things needed are the right ingredients. It’s bit like baking a cake; you can’t make a banana cake if you have no bananas or flour. Getting all the right ingredients Right here there is a major problem for chemical soup approaches to the origin of life: all the components have to be present in the same location for a living cell to have any possibility of being assembled. But necessary components of life have carbonyl (>C=O) chemical groups that react destructively with amino acids and other amino (–NH2) compounds. Such carbonyl- containing molecules include sugars,4 which also form the backbone of DNA and RNA. Living cells have ways of keeping them apart and protecting them to prevent such cross-reactions, or can repair the damage when it occurs, but a chemical soup has no such facility. Cells are incredibly complex arrangements of simpler chemicals. I am not going to cover every chemical that a first cell would need; it would take a book and some to cover it. I am just going to highlight some of the basic components that have to be present for any origin of life scenario. a. Amino acids Living things are loaded with proteins; linear strings of amino acids. Enzymes are special proteins that help chemical reactions to happen (catalysts). For example, the enzyme amylase is secreted in our saliva and causes starch molecules from rice, bread, potatoes, etc., to break up into smaller molecules, which can be then be broken down to their constituent glucose molecules. We can’t absorb starch, but we are able to absorb glucose and use it to power our bodies. Some reactions necessary for life go so slowly without enzymes that they would effectively never produce enough product to be useful, even given billions of years.5 Other proteins form muscles, bone, skin, hair and all manner of the structural parts of cells and bodies. Humans can produce well over 100,000 proteins (possibly millions; nobody really knows exactly how many), whereas a typical bacterium can produce one or two thousand different ones. Figure 1. Leucine, the most common amino acid, which is a specific arrangement of atoms of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). Proteins are made up of 20 different amino acids (some microbes have an extra one or two). Amino acids are not simple chemicals and they are not easy to make in the right way without enzymes (which are themselves composed of amino acids); see Figure 1. The 1953 Miller–Urey experiment, which almost every biology textbook still presents, managed to make some amino acids without enzymes. It is often portrayed as explaining ‘the origin of life’, but that is either very ignorant or very deceitful. Although tiny amounts of some of the right amino acids were made, the conditions set up for the experiment could never have occurred on Earth; for example, any oxygen in the ‘atmosphere’ in the flask would have prevented anything from forming. Furthermore, some of the wrong typesof amino acids were produced, as well as other chemicals that would ‘cross-react’, preventing anything useful forming. The amino acids required for functional proteins could never have been made by anything like this experiment in nature.6 When Stanley Miller repeated the experiment in 1983 with a slightly more realistic mixture of gases, he only got trace amounts of glycine, the simplest of the 20 amino acids needed.7 The origin of the correct mix of amino acids remains an unsolved problem (and see another major problem under ‘handedness’ below). Figure 2. Glucose, linear form b. Sugars Some sugars can be made just from chemistry without enzymes (which are only made by cells, remember). However, mechanisms for making sugars without enzymes need an alkaline environment, which is incompatible with the needs for amino acid synthesis. The chemical reaction that is proposed for the formation of sugars needs the absence of nitrogenous compounds, such as amino acids, because these react with the formaldehyde, the intermediate products, and the sugars, to produce non-biological chemicals. Ribose, the sugar that forms the backbone of RNA, and in modified form DNA, an essential part of all living cells, is especially problematic. It is an unstable sugar (it has a short half-life, or breaks down quickly) in the real world at near-neutral pH (neither acid nor alkaline).8 c. The components of DNA and RNA How can we get the nucleotides that are the chemical ‘letters’ of DNA and RNA without the help of enzymes from a living cell? The chemical reactions require formaldehyde (H2C=O) to react with hydrogen cyanide (HC≡N). However, formaldehyde and cyanide (especially) are deadly poisons. They would destroy critically important proteins that might have formed! Figure 3. Cytosine, one of the simpler of the five nucleotides that make up DNA and RNA. In this form of chemical diagram, each unlabelled bend in the ring has a carbon atom at the bend. Cytosine (Figure 3), one of the five essential nucleotide bases of DNA and RNA, is very difficult to make in any realistic pre-biotic scenario and is also very unstable.8 DNA and RNA also have backbones of alternating sugars and phosphate groups. The problems with sugars are discussed above. Phosphates would be precipitated by the abundant calcium ions in sea water or cling strongly onto the surfaces of clay particles. Either scenario would prevent phosphate from being used to make DNA. d. Lipids Lipids (‘fats’) are essential for the formation of a cell membrane that contains the cell contents, as well as for other cell functions. The cell membrane, comprised of several different complex lipids, is an essential part of a free-living cell that can reproduce itself. Lipids have much higher energy density than sugars or amino acids, so their formation in any chemical soup is a problem for origin of life scenarios (high energy compounds are thermodynamically much less likely to form than lower energy compounds). The fatty acids that are the primary component of all cell membranes have been very difficult to produce, even assuming the absence of oxygen (a ‘reducing’ atmosphere). Even if such molecules were produced, ions such as magnesium and calcium, which are themselves necessary for life and have two charges per atom (++, i.e. divalent), would combine with the fatty acids, and precipitate them, making them unavailable.9 This process likewise hinders soap (essentially a fatty acid salt) from being useful for washing in hard water—the same precipitation reaction forms the ‘scum’.
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